I'm thinking about the 12.9" but looking at both in the Apple Store has made me wonder whether the extra weight will annoy me. It'll mostly be for use at home sometimes for watching films/telly programmes on the sofa, sometimes as a second screen when I'm using my 13" MBP and sometimes to try to show pictures to my severely sight impaired mother. I'm thinking I'd use a stand for that last one (would need to even with the 11" as she's also found my old 9.7" iPad Pro too heavy to hold) so the weight issue is really for me on the sofa.
I guess maybe I should get one and see if it fits well into my life.